DELINEATION AND VULNERABILITIES OF HIV-1 ESCAPE FROM NEUTRALIZING ANTIBODIES ABSTRACT The development of effective interventions for human immunodeficiency virus type 1 (HIV-1) therapy and prevention is a high priority within the NIH mission. Recently identified broadly neutralizing antibodies (bNAbs) against HIV-1 envelope glycoproteins (Env) hold promise for future therapeutic and preventive interventions and may instruct efforts to develop an effective vaccine. Nevertheless, accumulating data have revealed the natural existence and rapid evolution of resistant strains that are no longer sensitive to different bNAbs. This problem is critical as the escape of HIV-1 isolates limits the coverage of any future vaccine that would rely on eliciting such bNAbs. Especially concerning are the potential development of multi-bNAb resistant strains and the spread of HIV-1 resistance in the viral population. Here, I am proposing a shift in the current scientific strategies to guide HIV-1 immunogen design. Our study will delineate resistance pathways used by HIV-1 to escape multiple bNAbs, identify new vulnerabilities of resistant HIV-1 strains, and develop new tools for the analysis of the balance between HIV-1 escape from bNAbs and replication fitness. In line with this RFA, we will specifically focus on HIV-1 strains that are resistant to multiple bNAbs among intravenous drug users (IDUs) and compare them to resistant strains from other populations. HIV-1 strains from IDUs exhibit high Env diversity that may facilitate the development of multi- bNAb resistance. Thus, elucidating the level and mechanisms of HIV-1 resistance to multiple bNAbs among viral strains from IDUs will be instructive for the future design of new Env immunogens that will be effective to IDUs. These insights will also aid selecting the optimal bNAbs for passive immunotherapy for this population. We will dissect the mechanisms of resistance, analyze the ability of resistant HIV-1 variants to replicate in vivo, and define the antigenic profile of bNAb-resistant strains. We will also build a database of HIV-1 strains that are resistant to bNAbs, solve the structure of Env of bNAb-resistant HIV-1 strains, and provide new tools to identify emerging patterns of resistance and assess the potential risk for resistance spread in the viral population. Altogether, this proposal represents a comprehensive approach to provide new tools and a critical knowledge base for the wider scientific community to aid the development of HIV-1 vaccines and passive immunization strategies that are beneficial to HIV-1 infected populations, including substance-using individuals.